1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly, to a semiconductor memory device used, for example, for storing graphic data, in which not only the data of a memory cell designated by an address but also the data of memory cells in the periphery of the memory cell designated by the address can be read out at the same time.
2. Description of the Prior Art
In graphic data processing, i.e., picture processing, effected in a computer aided design system (CAD), a graphic display apparatus, a television signal transmission system, and so on, a graphic data memory is used for storing graphic data being processed. The graphic memory often stores graphic data in correspondence to the graph or picture displayed by a graphic display apparatus and so on. In graphic processing, it is necessary to effect data processing on the graphic data stored in such a graphic memory, i.e., data processing such as compressing, differentiating, smoothing, and so on. To effect such data processing, it is necessary to read out and process the data not only of the designated memory cell but also of the memory cells in the periphery thereof. Therefore, it is desired that, in such a graphic memory and the like, not only the designated memory cell but also the memory cells in the periphery thereof can be quickly accessed.
However, in a conventional semiconductor memory device, since the data is read out from only one memory cell designated by an input address at the same time, it is necessary to access the memory device many times when graphic data processing is effected. Therefore, the sequence of the graphic data processing becomes complex and the processing time becomes very long
There is known another type of semiconductor memory device in which data is read out in parallel from a plurality of memory cells, for example, four memory cells, included in each cell group, for example, in each word, at the same time.
However, in such a memory device, the memory cells from which data can be read out in parallel are limited to those included within each cell group, and it is impossible to read out data simultaneously from memory cells belonging to different cell groups.